In patent document 1, an inductive electric power transfer circuit for an electric vehicle is proposed. That transfers electric power from a primary winding of a power supply facility to a secondary winding of a powered device of an electrical vehicle. Since it has no contact points, the inductive electric power transfer circuit has an advantage that is free from lose contact problems for electric terminals. The inductive electric power transfer circuit transfers power by electromagnetic induction on the secondary winding of the powered device with electromagnetic field generated by current on the primary winding of the power supply facility. A capacitor is connected between both ends of the secondary winding, and a resonant circuit is constructed with the secondary winding and the capacitor. The resonance of the resonant circuit strengthens the power received by the secondary winding. The power is transferred to the powered device which is connected to the resonant circuit in parallel. Some applications of this kind of inductive electric power transfer circuit that transfers electric power through space are toothbrushes, cellular phones, etc. The circuit according to patent document 1 keeps distance constant between the primary winding and the second winding of the powered device, and the powered devices such as toothbrushes or cellular phones are held at fixed positions in their holders to receive electric power. However, there is a deficiency that their power transfer efficiency according to patent document 1 becomes low when the distance between the primary winding and the secondary winding is enlarged, then mutual inductances M between the primary winding and the secondary winding becomes smaller than that in closed coupled mutual inductive circuits.
Patent document 2 provides a circuit that transfers electric power from a reader/writer, which is a power supply device with a primary winding, to an IC card, which is a powered device (remote device) with a secondary winding, on condition that distance between the primary winding and the secondary winding, which is far from the primary winding, changes. A capacitor is connected between both ends of the secondary winding of the powered device. The capacitor and inductance of the secondary winding construct a resonant circuit to increase electric power received. The power supply device is composed of a power supply circuit, a matching circuit, and a primary winding to adapt change of power transferred from the power supply device to the remote device when distance between the primary winding and the secondary winding changes. The remote device of IC card has a variable impedance circuit which is connected in parallel with the resonant circuit that is composed of the capacitance and the secondary winding. The resonant circuit is connected with a rectifier circuit, which is connected with a load circuit of IC chip. The variable impedance is adjusted to stabilize voltage on the load circuit by detecting the voltage. The power supplied to the load circuit is stabilized by this circuit, but there is a problem that the electric power supplied from the power supply circuit is uselessly consumed yielding low power transfer efficiency.
Patent document 3 provides a circuit which has improved power transfer efficiency from a reader/writer to an IC card by using a impedance varying means which varies two variable circuit elements, such as two variable capacitors, or as a variable capacitor and a variable inductance, by sensing power transfer efficiency with a sensing means.
Patent document 4 provides a circuit which has improved power transfer efficiency across wide space from a power supply facility to a remote device. The power supply facility adjusts supply of power according to information sent from the remote device that reports received power. That is, the electric power was supplied from the power supply to the remote device with high power transfer efficiency by varying two variable parameters of two circuit elements, capacitor and inductor in the power supply.    Patent document 1: WO92/017929    Patent document 2: JP10-145987    Patent document 3: JP2001-238372    Patent document 4: WO04/073166